This correspondence between Pascal and Fermat influenced
Christiaan Huygens, who published the first book on probability
theory three years later. Huygens was the guy who first declared
that Saturn has a ring, and who invented the pendulum clock.
Fermat was the first to evaluate the integral of general
power functions, which influenced both Newton and Leibnitz in
their creation of calculus.
See also in The book of science:
Readings on wikipedia: